Atlantic halibut (Hippoglossus hippoglossus)

The Atlantic halibut is classified as Endangered (EN) on the IUCN Red List (1).

A highly sought after food fish, the Atlantic halibut (Hippoglossus hippoglossus) is the largest flatfish in the world (2) (3). Flatfish exhibit a unique and distinctive anatomy that is adapted to their life on the sea bed; namely, they are flattened sideways and habitually lie on one side of their body, instead of being flattened from top to bottom like many others of the sea bed. As a result, both eyes tend to migrate to one side of the head during development. The Atlantic halibut lies on its left side and has both eyes positioned on its right, facing upwards (2). The fish is greenish-brown to dark brown or black on its upper surface and a dirty white on its lower surface (4) (5). Young fish are paler with more mottled colouration (3) (5).

Found in the cold waters of North Atlantic coasts (6), the Atlantic halibut is found in the Eastern Atlantic from the Bay of Biscay to Spitsbergen, Barents Sea, Iceland and eastern Greenland, and in the Western Atlantic from south-western Greenland and Labrador in Canada to Virginia in the USA (5).

This marine fish usually lives on the ocean floor at depths of around 50 to 2000 metres, but the Atlantic halibut occasionally comes closer to the surface (5). Larvae are pelagic, drifting relatively helplessly, but at around four centimetres they migrate to the bottom (3) (4). Young between two and four years live close to the shore, moving into deeper waters as they grow (4).

The Atlantic halibut has a relatively slow growth rate and late onset of sexual maturity (5), with males attaining maturity at seven to eight years old, females at ten to eleven years (4), and individuals are thought to live up to 50 years (3). Little is known about their breeding except that spawning is seasonal, although its timing varies somewhat with location. In the eastern Atlantic spawning occurs chiefly in March, April and May, although may span from January to June. Off the American coast, however, the spawning season appears to continue through the summer as late as September (3). After spawning both sexes migrate northwards in search of food (4). Young Atlantic halibut individuals feed on crustaceans like crabs and prawns, but older fish feed more on other fish, such as cod, haddock, herring and skate (4) (5). These halibut lie motionless and invisible on the sea bed, capturing any fish that pass within reach (3), although they may also hunt for fish in open water (4).

The Atlantic halibut has suffered massive declines throughout its range over the last two centuries, including virtual elimination in many areas as a result of over-fishing (7). Their slow growth rate and late onset of sexual maturity make these fish extremely vulnerable to the effects of over-fishing (5). Not only does this mean that individuals are often harvested many years before reaching maturity, and therefore unable to increase abundance through reproduction, but also that populations will be slow to recover from collapses in numbers (7). Since population numbers are now too low to sustain target fisheries, Atlantic halibut are predominantly taken as bycatch by bottom trawlers and longliners (8). Surveys indicate that these fish have continued to decline in the North Atlantic over the past two decades, despite being taken only incidentally as bycatch, with little targeted halibut fishing (7).

There is currently no management plan in place for this fish and it is therefore thought probable that numbers of Atlantic halibut will continue to decline. It has been argued that Atlantic halibut are unlikely to recover simply by banning halibut landings or designating protected areas. Rather, the recovery and survival of this Endangered flatfish species will depend on reducing its bycatch in other highly exploited fisheries (7).

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Bycatch: in the fishing industry, the part of the catch made up of non-target species.

Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps and barnacles.

Larvae: stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.

Pelagic: inhabiting the open oceans, away from the ocean bottom.

Spawning: the production or depositing of large quantities of eggs in water.